Youngest Billionaires In The United States - 2012 Forbes 400

If the Forbes 400 is the ultimate scorecard of wealth, these 20 people may be winning — even more than Bill Gates or Warren Buffett.

While Gates and Buffett have the edge in total dollars, it's the young billionaires that really get to have all the fun. Rather than planning retirement and succession, these folks are starting new companies and families, searching for new businesses to disrupt and new passions to engage.

This year's group of 20 youngest billionaires in America makes up just under $100 billion in total net worth ($98.6 billion to be precise). Dustin Moskovitz, at 28 years old, holds the title of youngest billionaire in the U.S. The Facebook cofounder made the list at $2.7 billion, down from $3.5 billion a year ago. Mark Zuckerberg, the second-youngest, also saw his net worth drop off a cliff after the Facebook IPO flop. Zuckerberg, who's only eight days older than his former college roommate Moskovitz, had an estimated net worth of $17.5 billion in 2011. Now he's down to $9.4 billion — certainly nothing to cry about (even though in dollar terms he's the year's biggest loser).

Silicon Valley is well represented among the young and wealthy. Including Moskovitz and Zuckerberg, half of the top 20 youngest billionaires built their fortune in tech. Sean Parker, 32, is a Facebook alumnus who's moving forward with video chat platform Airtime. Jack Dorsey, 35, is the chairman of Twitter and CEO of Square — he's a newcomer to the 400 list at $1.1 billion. Then there are the 39-year-old Google founders, Larry Page and Sergey Brin. Combined, the duo is worth more than $40 billion in Google stock.

Veterans from the first tech boom also dot the list. Jerry Yang, 42, founded Yahoo 18 years ago. He's still going strong at $1.2 billion, although his company is looking for its latest rebirth under new CEO Marissa Mayer. Pierre Omidyar, who started eBay, is more focused on philanthropy, while PayPal founder Elon Musk has put his money into electric cars (Tesla) and space travel (SpaceX).

But tech titans aren't the only ones making money at an early age. More than a few hedge fund whiz kids are enjoying success. Chase Coleman and John Arnold have already amassed billion-dollar funds. Coleman, 37, shot to the top of the hedge fund heap — his Tiger Global beat nearly all other hedge funds in 2011 with a 45 percent return. Meanwhile, Arnold has already cashed in his chips. He shocked the investing world when he announced his retirement at the precocious age of 38. He now plans to devote much of his time (and $3 billion net worth) to charitable pursuits.

Two others who made the youngest list last year but dropped off are Zynga founder Mark Pincus, 46, who lost more than half of his net worth as the stock tanked, and Facebook cofounder Eduardo Saverin, 30, who renounced his U.S. citizenship in 2011 and permanently relocated to Singapore.